Quantitiate the in vivo mammary tumor onset and incidence in the following experimental situations. For a 12 month period in virgin Blab/c animals primed and unprimed with hormones from pituitary implants. For a 12 month period following implantation of expanded hyperplastic cell populations in virgin Balb/c animals primed and unprimed with hormones from ptuitary implants. Determine if MTV RNA is induced in host mammary cells during malignant transformation by hormones, and if the induction correlates with tumor onset and incidence in vivo. Determine if changes in expression of MTV structural plypeptides occur diring transformation by hormones and if those changes correlate with tumor onset and incidence. Determine if new MTV sequences are added to mammary cell DNA during transformation by hormones. Identify individual mammotropic hormones which cause the same changes in MTV expression as those which occur during hormonal insults of proven but undefined oncogenicity.